M4 Sherman with composite plastic armor

 HCR2 for M4 Sherman tanks, one of the first composite armors.

The devastating effect of the lightweight German hollow charge weapons such as the Panzerfausts and Panzerschrecks (encountered by US forces in the European theater) re-sulted in all sorts of field improvisations to strengthen the Shcrman's armor. At the same time. Ordnance initiated several programs to develop protection a-gainst these weapons.Much of the test work extended into the postwar period.

One such project evaluated the use of plastic armor added to the sides of the hull and turret. One of the experiments showed that plastic armor was able to achieve the same level of protection as steel but with a much lighter weight.

The armor was further improved through a series of tests and resulted in two variants, the HCR1 and HCR2.

HCR1 was composed of:

• 50% aluminium fillings

• 40% asphalt or pitch

• 10% wood flour

HCR2 was composed of:

• 80% gravel

• 5% wood flour

• 15% asphalt or pitch

HCR2 proved to have the best performance and the original plan for tank protection with plastic armour was to produce HCR2-filled steel panels that could be readily fitted to Allied tanks. However, this project was abandoned due to the low probability of these weapons becoming a serious threat.

The HCR2, was a mixture of quartz gravel and a mastic composed of asphalt and wood flour. A kit of auxiliary plastic armor was designed by the Flintkote Company for the M4 series of tanks. The armor panels consisted of 10 inches of HCR2 faced with 1 inch  aluminum plate. These panels were held in place over the sponsons and around the turret by 1/2 inch steel cables attached to brackets welded to the tank. The plastic armor kit increased the weight of the vehicle by about 8 tons. Tests at Aberdeen in the Fall of 1945 showed that the turret assembly could defeat the German Panzerfaust 100 or the 8.8cm rocket, but both weapons caused some penetrations on the sponsons. The plastic armor also increased the protection against the kinetic energy projectiles such as the 76mm H VAP, but not as much as an equivalent weight of steel. It was also noted that the attachment cables were vulnerable to attack by high explosive rounds sometimes resulting in the complete loss of an armor panel.